Radio communication devices use antennas to provide for the efficient transmission of radio frequency (RF) communication signals. The transmitter portion of a radio communication device includes a power amplifier to amplify the RF signals before they are coupled to the antenna for transmission. For some modulation techniques, such as quadrature amplitude modulation (QAM), linear amplification is desired to prevent distortion of the modulated signal. However, when RF power amplifiers are operated in their most efficient manner at high drive levels, they usually provide a non-linear “compression” characteristic. This means that a change in the amplitude of a signal sent into the power amplifier results in a non-proportional change in the amplitude of the signal out of the amplifier, and therefore causes distortion of the signal. Non-linearities may also be caused by changes in load impedance, as may be caused by the operating environment of the power amplifier.
One manner of improving the linearity of a RF transmitter is to use a Cartesian feedback system, whereby a feedback signal path is provided to create a negative feedback which compensates for compression in the power amplifier. More particularly, in a typical Cartesian feedback system, a drive signal is input at baseband. The drive signal is a complex baseband signal having in-phase (I) and quadrature (Q) components. Each of the I and Q components are summed with a feedback signal and separately filtered and applied to a quadrature up-converter which translates the components to a RF frequency. The RF signal is then amplified by a power amplifier and sent to an antenna for transmission. To create a feedback loop, the output from the power amplifier is fed back to a quadrature down-converter that translates the RF signal to a pair of baseband signals, which are then summed with the original drive signal.
In such systems, a correct phase relationship is required between the local oscillator (LO) signals that are used for driving the quadrature up-converter and down-converter. To set the correct phase relationship, a phase correction is typically performed first in a phase training mode and then updated as necessary in a circulator elimination (CE) mode. In the phase training mode, phase correction is performed by inserting a training signal between the output of a forward path filter and the input to the quadrature up-converter (also known as the LP2 point), shutting off the gain in the baseband input paths, and breaking the loop to maintain the bias of the system. The output of the summing junction for the input baseband signal and the feedback loop is monitored, and an algorithm is used to adjust the phase of one of either the up-converter LO signals or the down-converter LO signals relative to the other until the input I component is aligned with the I channel feedback and the input Q input is aligned with the Q channel feedback to establish correct negative feedback.
In CE mode, the loop is closed and the gain for the baseband input paths is turned on. During operation, the signal from the LP2 point is digitized and sent to a digital signal processor (DSP). The DSP then continuously makes incremental changes to the phase of either the up-converter LO signals or down-converter LO signals based on the relationship between the input signal and the LP2 signal.
However, there are numerous disadvantages to such Cartesian feedback systems. For example, when used with multi-band communication devices, multiple RF filters are required to enable LO signal phase shifting for the multiple bands. Such filters are both large and expensive, even when they are fabricated on an integrated circuit and are therefore undesirable. As phase changes implemented in present systems also result in modulation of the desired output signal, such Cartesian feedback systems also result in a significant amount of off-channel splatter during the incremental phase adjustments performed during the CE mode.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions and/or relative positioning of some of the elements in the figures may be exaggerated relative to other elements to help improve the understanding of various embodiments of the present disclosure. Also, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are not often depicted in order to facilitate a less obstructed view of these various embodiments of the present disclosure. It will be further appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meaning have otherwise been set forth herein.